Not an ST400. Those had longer chain-stays. That's the race frame.

Yes, you're reading the serial number correctly, but I also don't know what the 4 means for sure. The scheme matches that of my 1987 R400. The previous owner did me the favor of upgrading it all to Shimano 600.

Yes, these bikes had short wheelbases. My 60cm R400/600 is slightly shorter in wheelbase than my 2013 CAAD10 (as noted every time I load it on my roof rack). Old bike still rides great, though.